1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to the field of providing test access to integrated circuits, such as, for example, debug access via serial scan chains and trace access via embedded trace units.
2. Description of the Prior Art
As integrated circuits and system-on-chip designs have developed in size and complexity, the importance of thorough, quick and convenient testing tools for those designs and systems increases in importance. A significant amount of time in the development of a new integrated circuit, system-on-chip circuit or system product is devoted to the testing, validation and debugging of the design.
As an example, in the field of mobile telephones it is known to provide debug and trace test access into the telephone systems to allow these to be tested and debugged. These testing interfaces sometimes remain in place in the shipped products and can be used to diagnose additional problems when units are returned from field service.
Within the field of integrated circuits and data processing systems in general there is an increasing need for secure operation e.g. smart cards bearing integrated circuits for controlling financial transactions where the user needs to be authenticated and/or data needs to be encrypted/decrypted. As another example, copyright material owners require the use of decryption keys obtained by payment in order to access their copyright material. It is critical that the decryption keys remain secure and cannot be accessed by unauthorised persons. Accordingly, considerable effort is devoted to developing systems with secure modes of operation and that resistant to unauthorised intrusion which so as to keep secret data and operations secure. Smartcards for use in financial transactions are another example of integrated circuits with a requirement for a high degree of security.
It is known from British Published Patent Application GB-A-2,395,583 to provide an integrated circuit having a hardware control pin (JSDAEN) which it is assumed is tied down to ground (i.e. set low) in production. This assumption applies only to certain production scenarios—alternative scenarios require the pin to be set high or low according to the category of s/w developer, with all developers using standard production units. This is a potential security vulnerability.